The present invention relates to mobile radio communication systems and, in particular, to a mobile radiotelephone method and system in which a geographical area is provided with mobile radiotelephone service by dividing the geographical area into smaller geographical areas through the use of directional and omnidirectional antennas.
Approximately twelve communication channels, each including two distinct frequencies for two-way communications (an up-link and a down-link), are typically available for use in present day mobile telephone systems. In one known system, usually referred to as Improved Mobile Telephone Service (IMTS), communication is established over the available channels from a wire line telephone system to mobile units through the use of transmitters which transmit omnidirectionally throughout a large geographical area or zone from an antenna approximately centered in the area. A plurality of satellite receivers are spaced throughout the geographical area to receive transmissions from the mobile units and relay the transmissions to the central location. Calls are established through seizure of a marked idle channel by a mobile unit and by placing or receiving a call over the seized marked idle channel.
With only twelve channels available for use in this type of system, only twelve simultaneous conversations are possible and, not only is the total number of subscribers in a service area necessarily limited, but also the subscribers who do obtain service tend to find circuits busy a very high percentage of the time when attempting to place a call. It can thus be seen that 12-channel systems based upon a large zone coverage from a central location have been unable to meet present user demands, let alone future requirements for mobile radiotelephone service. As a result, several different arrangements have been employed in an attempt to increase the capacity of a system.
One attempt to improve channel availability is to provide frequency reuse of the existing channels by changing radio coverage philosophy from that of high power transmission, long range reception to low power transmission, short range reception. This type of radio coverage is typically referred to as a "small zone" concept in which small zones of reception and transmission are defined by spaced based stations. In a small zone system, channel reuse is permissible where sufficient spacing exists between two small zones, and such reuse makes improved channel availability possible.
For example, increased capacity may be attained by dividing a large service area or zone into small service areas or zones with a base station at the center of each small zone. Transmitters and receivers at each base station together provide mobile telephone coverage of the large geographical area. In such an arrangement, channels used in one small zone may be reused in another small zone if sufficient spacing exists between the two zones. Thus, in a large metropolitan area, for example, available channels may be reused in several separated small zones with a minimum of interference.
The equipment requirements in a small zone system may greatly exceed that required in a large zone system but the increased system capacity may more than balance this increased expense. Another drawback in a typical small zone system is that the mobility of a mobile telephone unit may also be somewhat decreased due to the movement of a mobile unit from one small zone into another and the resultant loss of the signal channel over which a call is established.
A further problem in known mobile telephone systems is the problem of increasing area coverage as the outer limits of the service area expand. Relatively facile expansion of the mobile telephone system as a service area grows in size is extremely important both in efficiently employing the limited channels presently allocated to mobile telephone service and in making proper use of the vastly increased number of channels which may become available in the UHF spectrum.
It is accordingly an object of the present invention to provide a novel method and mobile communication system not limited to a small zone concept which obviates these and other problems associated with known mobile telephone systems.
It is another object of our invention to provide an improvement in mobile communication systems as the result of the combined use of an omnisectional array (OSA) antenna, sector area beacon transmitters, sector area monitor receivers, and a central processor, thus making a greater degree of re-use of frequency channels possible.
It is a further object of the present invention to provide a novel method and mobile telephone system for providing significant frequency re-use, thus more efficiently employing the communication channels allocated to mobile telephone service in a manner compatible with existing mobile telephone equipment.
It is another object of the present invention to provide a novel method and mobile telephone system which is readily expandable to accommodate the growth of a service area.
It is yet another object of the present invention to provide a novel method and mobile telephone system which employs a combination of directional and onmidirectional antennas to increase mobility of mobile units operating within a service area and to minimize problems caused by back radiation from the directional antennas.
It is yet a further object of the present invention to provide a novel method and system for establishing mobile telephone communication within a predetermined zone or sector with a relatively high degree of zone specificity.
It is still a further object of the present invention to provide a novel method and system for providing two-way communication to and from mobile units in a manner both compatible with existing mobile communication equipment and readily expandable to accommodate system growth with the addition of fewer base stations than are required in prior art systems.
It is yet a further object of our invention to provide a novel frequency re-use capability in a land mobile radio system, accomplished at a greatly reduced cost, insofar as base station equipments and real property purchases are concerned, when compared with the cost of implementing a conventional small zone concept.
It is yet a still further object of our invention to provide a highly effective zone transfer capability in a mobile radio system, accomplished by determining the approximate location of a budy mobile unit, by measuring and comparing its signal strength as received at monitoring sites, and then assigning the proper transmitter frequency and base station site and sector via a central processor algorithm.
These and other objects and advantages are accomplished in accordance with the present invention as will become apparent to one skilled in the art to which the invention pertains from a perusal of the following detailed description and claims when read in conjunction with the drawings.